1. Field of the Invention
The present invention relates to a method for producing a lithographic printing plate and also relates to a lithographic printing plate precursor and a lithographic printing method. More specifically, the present invention relates to a method for producing a lithographic printing plate by removing, after exposure, the unexposed area of the image recording layer with use of a developer at a pH of 2 to 10 in an automatic processor, and also relates to an on-press development type lithographic printing plate precursor and a lithographic printing method.
2. Background Art
The lithographic printing plate in general consists of a lipophilic image area of receiving an ink and a hydrophilic non-image area of receiving a fountain solution in the printing process. The lithographic printing is a printing method utilizing the repellency between water and oily ink from each other, where the lipophilic image area and the hydrophilic non-image area of the lithographic printing plate are formed as an ink-receiving part and a fountain solution-receiving part (ink non-receiving part), respectively, to cause difference in the ink adhesion on the surface of the lithographic printing plate, an ink is attached only to the image area and thereafter, the ink is transferred to a material on which the image is printed, such as paper, thereby performing printing.
For producing this lithographic printing plate, a lithographic printing plate precursor (PS plate) comprising a hydrophilic support having provided thereon a lipophilic photosensitive resin layer (image recording layer) has been heretofore widely used. Usually, a lithographic printing plate is obtained by a plate-making method where the lithographic printing plate precursor is exposed through an original image such as lith film and while leaving the image recording layer working out to the image area, the other unnecessary image recording layer is dissolved and removed with an alkaline developer or an organic solvent to reveal the hydrophilic support surface, thereby forming a non-image area.
In the plate-making process using a conventional lithographic printing plate precursor, a step of dissolving and removing the unnecessary image recording layer with a developer or the like must be provided after exposure but as one problem to be solved, it is demanded to simplify such an additive wet processing. As one technique for the simplification, capability of development with an aqueous solution close to neutral or with mere water is demanded.
On the other hand, a digitization technique of electronically processing, storing and outputting image information by using a computer has been recently widespread and various new image-output systems coping with such a digitization technique have been put into practical use. Along with this, a computer-to-plate technique is attracting attention, where digitized image information is carried on a highly converging radiant ray such as laser light and a lithographic printing plate precursor is scan-exposed by this light to directly produce a lithographic printing plate without the intervention of a lith film. Accordingly, one of important technical problems to be solved is to obtain a lithographic printing plate precursor suitable for such a technique.
Under these circumstances, the demand for plate-making work satisfying both simplification and digitization is ever-stronger at present.
For example, Patent Document 1 (JP-A-2002-365789 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)) describes a lithographic printing plate precursor comprising a hydrophilic support having thereon an image forming layer containing a hydrophobization precursor, a hydrophilic resin and a light-to-heat converting agent, wherein the image forming layer further contains a compound having an ethylene oxide chain, so that on-press development and also after exposure and liquid development with a developer which is water or an appropriate aqueous solution, printing can be performed.
Patent Document 2 (U.S. 2004/0013968) describes a method for processing a lithographic printing plate precursor, wherein a lithographic printing plate precursor comprising (i) a hydrophilic support and (ii) a lipophilic thermosensitive layer containing a radical polymerizable ethylenically unsaturated monomer, a radical polymerization initiator and an infrared absorbing dye, which is cured by exposure with an infrared laser and can be developed with an aqueous developer containing 60 mass % or more or water and having a pH of 2.0 to 10.0, is prepared and imagewise exposed with an infrared laser, and the uncured region of the thermosensitive layer is removed with an aqueous developer.
On the other hand, in a lithographic printing plate precursor utilizing a polymerization reaction, a protective layer (overcoat layer) is usually provided so as to block oxygen which inhibits the polymerization reaction, or prevent scratching of the image recording layer or ablation at the exposure with a high-intensity laser. Here, in the protective layer, a water-soluble polymer compound with excellent crystallinity, such as polyvinyl alcohol, has been heretofore used as the main component. Also, Patent Document 3 (JP-A-11-38633) describes a radical polymerization-type lithographic printing plate precursor comprising a protective layer containing an inorganic layered compound such as mica and states that this lithographic printing plate precursor ensures high sensitivity and good storage stability and can be developed with an aqueous alkali solution.
As another simple and easy plate-making method, a method called on-press development has been proposed, where an image recording layer allowing for removal of the unnecessary portion of the image recording layer during a normal printing process is used and after exposure, the unnecessary portion of the image recording layer is removed on a printing press to obtain a lithographic printing plate.
Specific examples of the on-press development method include a method using a lithographic printing plate precursor having an image recording layer dissolvable or dispersible in a fountain solution, an ink solvent or an emulsified product of fountain solution and ink, a method of mechanically removing the image recording layer by the contact with rollers or a blanket cylinder of a printing press, and a method of weakening the cohesion of the image recording layer or adhesion between the image recording layer and the support by the impregnation of a fountain solution, an ink solvent or the like and then mechanically removing the image recording layer by the contact with rollers or a blanket cylinder.
For example, Patent Document 4 (Japanese Patent 2,938,397) describes a lithographic printing plate precursor where an image forming layer comprising a hydrophilic binder having dispersed therein hydrophobic thermoplastic polymer particles is provided on a hydrophilic support. In Patent Document 4, it is stated that after this lithographic printing plate precursor is exposed with an infrared laser to cause thermal coalescence of hydrophobic thermoplastic polymer particles and thereby form an image, the printing plate precursor can be loaded on a cylinder of a printing press and then on-press developed by supplying a fountain solution and/or ink.
Such a method of forming an image through coalescence by mere heat fusion of fine particles has a problem that despite good on-press developability, the image strength is extremely low and the press life is insufficient.
For solving these problems, a technique of improving the press life by utilizing a polymerization reaction has been studied. For example, Patent Documents 5 and 6 (JP-A-2001-277740 and JP-A-2001-277742, respectively) describe a lithographic printing plate precursor comprising a hydrophilic support having thereon a layer containing a polymerizable compound-enclosing microcapsule. Also, Patent Document 7 (JP-A-2002-287334) describes a lithographic printing plate precursor comprising a support having provided thereon a photosensitive layer containing an infrared absorbent, a radical polymerization initiator and a polymerizable compound. Furthermore, Patent Document 8 (JP-A-2000-39711) describes a lithographic printing plate precursor which can be on-press developed after exposure, the printing plate precursor comprising an aluminum support having thereon a layer comprising a photosensitive composition containing (a) a water-soluble or water-dispersible polymer, (b) a monomer or oligomer having a photopolymerizable ethylenically unsaturated double bond and (c) a photopolymerization initiation system having absorption maximum in the ultraviolet region.
In the case of developing a negative lithographic printing plate precursor having a protective layer, the image recording layer in the unexposed area and also the protective layer must be removed by the development. However, if the image recording layer in the unexposed area and the protective layer are removed at the same time, the protective layer components dissolved in the developer may cause a problem such as reduction in the activity of developer or generation of scum in the developer.
In order to solve this problem, in the case of a conventional image recording layer developable with an alkaline developer, it may be possible to previously remove the protective layer by water washing and then remove the image recording layer in the unexposed area by development, but when a non-alkaline developer preferred in view of working safety is used, the protective layer as well as the unexposed area of the image recording layer are removed by water washing and it is difficult to separately perform the removal of protective layer and the development of image recording layer.
Furthermore, in the on-press development type lithographic printing plate precursor, a protective layer is usually provided on the image recording layer so as to prevent scratching or staining of the image recording layer, block oxygen or prevent ablation at the exposure with a high-intensity laser, and in the protective layer, a water-soluble polymer compound with excellent crystallinity, such as polyvinyl alcohol, has been heretofore used as the main component. The water-soluble polymer compound with excellent crystallinity readily remains due to its low dissolution rate in the fountain solution and this disadvantageously deteriorates the inking property or on-press developability. Also, the fine line reproducibility is still insufficient.